1. Field of the Invention
The present invention relates to a high-voltage output circuit using a piezoelectric transformer, a high-voltage power supply device having the high-voltage output circuit, and an image forming apparatus having this high-voltage power supply device.
2. Description of the Related Art
In recent color laser printers and the like, a plurality of photosensitive members provided for the respective colors are scanned independently by light beams from a plurality of respective optical units, thereby forming images of each of the colors. The color images are superimposed on an intermediate transfer belt and then transferred from the belt to a printing paper. Such a system is referred to as a “tandem system”. Since this system forms toner images of a plurality of colors concurrently, superimposes the image on the intermediate transfer belt and then transfers the resultant color image to a printing paper, the time it takes to form the final color image can be shortened greatly.
In an electrophotographic process employed by such a color laser printer, DC voltage is used for charging bias applied to a charging roller, developing bias applied to a developer and transfer bias applied to a transfer roller. High voltage is required for these bias voltages. For example, in order to achieve good transfer in the case of transfer bias, usually a DC voltage of 3 kV or greater is required.
Conventionally, a wire-wound electromagnetic transformer is used in order to generate high voltage in a laser printer. Such an electromagnetic transformer is composed of copper wire, a bobbin and a magnetic core. In a case where a voltage of 3 kV or greater is applied using this transformer, leak current must be minimized because of a very small current the output current value of which is several microamps. For this reason, it is required that the transformer windings be molded in an insulator. This necessitates a transformer that is large in comparison with the supplied power. This is an obstacle in terms of reducing the size and weight of the high-voltage power supply device.
Accordingly, in order to achieve a reduction in the size and weight of a high-voltage power supply device, a high-voltage power supply device that generates high voltage using a thin, light-weight, high-output piezoelectric transformer (piezoelectric ceramic transformer) has started to be employed. By using a piezoelectric element made of a ceramic material as the piezoelectric transformer, it is possible to generate high voltage at an efficiency greater than that of the electromagnetic transformer. Moreover, since it is possible to increase the distance between the electrodes on the primary and secondary sides irrespective of the coupling between the primary and secondary sides, special molding work is not required for the purpose of insulation. An advantage is that the high-voltage power supply device can be reduced in size and weight, and the reduction in the size of the apparatus is great in comparison with the case where the wire-wound electromagnetic transformer is used. An example such a high-voltage power supply device using a piezoelectric transformer is disclosed in, e.g., Japanese Patent Laid-Open No. 11-206113.
FIG. 12 is a block diagram illustrating an example of a conventional high-voltage power supply device using a piezoelectric transformer. FIG. 12 illustrates an example of a circuit that outputs negative bias (negative high voltage).
Specifically, the circuit includes a piezoelectric transformer 101 for a high-voltage power supply. The output of the piezoelectric transformer 101 is rectified and smoothened to a negative voltage by diodes 102, 103 and a high-voltage capacitor 104, and the negative voltage is supplied from an output terminal 116 to a charging roller (not shown), which is the load. Further, the output voltage is potential-divided by resistors 105, 106, 107 and applied to a non-inverting input terminal (+ terminal) of an operational amplifier 109 via a protective resistor 108. A control signal Vcont, which is an analog signal for controlling the high-voltage power supply, is input via a connection terminal 118 from a controller (not shown) of a printer that accommodates this high-voltage power supply device. The control signal Vcont is applied to an inverting input terminal (− terminal) of the operational amplifier 109. An integrating circuit is constructed by the operational amplifier 109, a resistor 114 and a capacitor 113.
The output of the operational amplifier 109 is connected to a voltage-controlled oscillator (VCO) 110, and the output of the VCO 110 is connected to a FET 111 connected to an LC parallel resonance circuit formed by an inductor 112 and capacitor 115. The frequency of the signal that is output from the VCO 110 rises when the input voltage to the VCO 110 rises and falls when the input voltage falls. Accordingly, the VCO 110 outputs a signal having a frequency conforming to the level of this input voltage. The output signal of the VCO 110 drives the above-mentioned LC resonance circuit, whereby a voltage conforming to the control signal Vcont is finally supplied to the primary side of the piezoelectric transformer 101.
FIG. 13 is a diagram illustrating the driving frequency vs. output voltage characteristic of the piezoelectric transformer 101.
As illustrated in FIG. 13, the piezoelectric transformer 101 has such a characteristic that output voltage peaks at a resonance frequency f0, and it will be understood that the output voltage can be controlled based upon frequency. For example, let fx represent driving frequency at output of a stipulated output voltage Edc. The VCO 110 is such that the output frequency thereof varies in accordance with the control signal Vcont, as mentioned above. Accordingly, in a case where the piezoelectric transformer 101 is controlled so as to obtain an output voltage higher than the output voltage Edc, the output frequency of the VCO 110 is made a frequency lower than fx. In a case where the piezoelectric transformer 101 is controlled so as to obtain an output voltage lower than the output voltage Edc, the output frequency of the VCO 110 is made a frequency higher than fx. That is, the circuit depicted in FIG. 12 constructs a negative feedback control circuit in which the output voltage at the output terminal 116 is subjected to constant-voltage control in such a manner that the output becomes equal to a voltage decided by the voltage of the control signal Vcont applied to the inverting input terminal (− terminal) of the operational amplifier 109.
With the high-voltage power supply device of piezoelectric transformer type according to the prior art described above, however, a certain problem arises.
Specifically, when various components are solder-mounted automatically on a circuit board of a high-voltage power supply device of piezoelectric transformer type, an excessive voltage is produced across the terminals owing to the pyroelectric effect of the piezoelectric transformer 101 ascribable to the heat of a solder tank (solder bath). As consequence of the excessive voltage, the voltage applied to the FET 111 for driving the piezoelectric transformer 101 connected via the wiring on the board exceeds the withstand voltage of the FET, thereby destroying the FET 111.
In order to deal with this problem, the high-voltage power supply device of piezoelectric transformer type is such that when parts are solder-mounted on the board, components other than the piezoelectric transformer 101 are solder-mounted automatically using the solder tank, after which the piezoelectric transformer 101 is soldered to the board manually. However, a large number of piezoelectric transformers are mounted on the circuit board of a high-voltage power supply device used in a tandem-type color laser printer, the manual soldering process is time consuming and solder-mounting cost rises. Further, since the soldering operation is performed manually, solder-mounting errors are likely to occur and hence there are limitations in terms of improvements in yield and productivity.